<ribbon:XamRibbonWindow
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WPFWindowHostWF"
xmlns:ribbon="http://infragistics.com/Ribbon"
xmlns:forms="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms"
xmlns:themes="http://infragistics.com/Themes"
x:Class="WPFWindowHostWF.MainWindow"
mc:Ignorable="d"
Title="MainWindow" WindowStyle="None"
SizeToContent="WidthAndHeight">
<ribbon:XamRibbonWindow.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<!-- First, import the base theme we are going to wash to ensure
the correct ControlTemplates are used-->
<!-- Add a ResourceWasher -->
<!-- Note:
1. The WashMode property defaults to "SoftLightBlend" but can be set to "HueSaturationReplacement"
2. If the WashColor of an individual WashGroup is not set or is left to its default value of "Transparent" then the resources are not washed
-->
<themes:ResourceWasher AutoWash="True" WashMode="HueSaturationReplacement" WashColor="Gray" >
<themes:ResourceWasher.SourceDictionary>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<themes:RibbonOffice2013/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</themes:ResourceWasher.SourceDictionary>
<themes:ResourceWasher.WashGroups>
<themes:WashGroupCollection>
<!-- Specify WashColors for each WashGroup in the Theme. The WashGroups are the same in all themes. -->
<themes:WashGroup Name="TabArea" WashColor="Red"/>
<themes:WashGroup Name="Pressed" WashColor="Red"/>
<themes:WashGroup Name="BaseColor" WashColor="Red"/>
</themes:WashGroupCollection>
</themes:ResourceWasher.WashGroups>
</themes:ResourceWasher>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</ribbon:XamRibbonWindow.Resources>

For gray background color the key is ContainerFillKey and for the tabs green caption are you can use CaptionPanelFillKey and RibbonTabControlHeaderPanelBackgroundKey keys available in the ‘RibbonGeneric.xaml’ file available on the same path.

I override the ‘RibbonTabItem Style’ style.

I am not sure if the ResourceWasher would work in case of transparent default color. So I would recommend you to achieve it by modifying the default style using resource dictionary.

Also for more style or color changes you can simply find the key using snoop.Snoop is very useful such scenario and then find the same key in the default theme file and override it into your ResourceDictionary, as I did.